This invention relates to a method for removing biofilm, debris, contaminants and the like from the surfaces of passageways, including passageways which have irregular or complex shapes or whose walls are permeable, using a mixed-phase cleaning solution.
Interior surfaces of passageways such as small-bore tubing, pipes, ducts and the like, which may carry fluids such as liquids, gases, slurries or aerosols, are very difficult to clean and to maintain in a clean condition. When the flow path is long and narrow, or hard to reach, it is difficult to clean the surfaces by conventional liquid phase flushing because such a long, narrow passageway limits liquid flow velocities by creating a high resistance to flow. As a result, shear stresses which could aid in the removal of contaminants from such surfaces are limited. Low flow velocities also limit the usefulness of solvents for the same reasons.
Cleaning of small diameter passageways is also difficult because of the nature of certain types of residues. Fluid passageways which supply water, even purified water, develop bacterial and fungal growth from the water on their interior surfaces, as is well known. Bacteria present in the water strongly adhere to tubing surfaces and then grow laterally, forming what is known as biofilm. Biofilm is apparent to the touch as a slimy film and is composed of both organic residues and the multiplying microorganisms. The bacteria deposit an underlying structural matrix comprising polysaccharides with some peptide moieties, calcium carbonate and other materials which adhere to the surfaces of the passageways. An illustrative example is dental unit water line tubing, which carries rinse water to the mouth of a dental patient. It has been determined that, in the absence of any special precautions, this water exiting from such tubing can include as much as one million (1xc3x97106) colony-forming units of bacteria per milliliter of water (CFU/ml). The source has been shown to be the surface biofilm which sheds bacteria into the flowing water. The American Dental Association has recommended reduction of the level of bacteria present in dental water delivery systems to below 200 CFU/ml to be adopted by the year 2000. Thus, these water lines and tubing must be periodically disinfected or cleaned to ensure the deactivation of viable bacteria and the removal of this biofilm from the walls of the tubing in order to prevent infection in dental patients. Removal of biofilm from passageways is also necessary for other applications, including medical, industrial and food service applications, because such biofilms are the main cause of high bacterial counts and high levels of endotoxins.
However, removing biofilm from fluid passageways is quite difficult, which makes disinfecting the surface more difficult as well. The biofilm is strongly adherent to passageway surfaces, whether the surface is made from natural materials, such as rubber or metals, or synthetic polymeric materials, such as polyvinylchloride, polyethylene, polytetrafluoroethylene and the like. Treatment with chemical agents, such as disinfectant and biocidal agents, can kill the exposed surface bacteria and so reduce the contribution of the biofilm to the total bacterial count. However, these agents do not readily diffuse into the entire thickness of the biofilm. The biofilm protects the remaining viable bacteria which then rapidly multiply. If it happens that all of the bacteria are killed, the biofilm structure remains an ideal host for new bacteria to colonize and grow. Thus these treatments are generally only partially effective, and the original levels of viable bacteria return quite rapidly. In order to remove biofilm from a surface, in addition to chemical treatment, some mechanical action is necessary to produce shear stress or sufficient impact at the surface.
In dentistry, there are applications for cleaning and disinfecting both tubing and the dental handpiece. The handpiece, which contains an air-driven turbine or other method of driving a drill and other parts, is about six inches long and is detachable. The tubing and other passageways inside the handpiece have a ratio of length to inside diameter of about 100. At present the most common sterilization procedure is steam autoclaving. However, in addition to the fact that autoclaving does not actually remove debris from the handpiece, this autoclaving procedure can be damaging to the turbine and various seals in the handpiece. For example, the operating rotational speed of a dental drill has been found to decrease with the number of sterilizations performed.
The old method of cleaning dental handpieces is to flush them with water or a cleaning solution. While this may flush nonadherent biofilm and debris from passageways, it can be shown that it provides little or no removal of adherent biofilm and debris such as blood, mucous and the like. In order to obtain more force behind the liquid flushing, Littrell, U.S. Pat. No. 3,625,231, describes a device utilizing compressed air to force a quantity of a cleaning and conditioning fluid through the passageways of the handpiece. This device primarily uses single-phase liquid flow as evidenced by the requirement to observe the clarity of the fluid being expelled from the handpiece as a criterion for cleaning. This method is only slightly more effective than flushing with water but may be significantly better than flushing with a hand-operated syringe. However, complete removal of adherent biofilm, debris and contaminants will not occur.
Cleaning of instruments, handpieces and the like by spraying with water or cleaning solutions is also well known. The spray may be generated by an aerosol can or an atomizing device. While this is a useful method of distributing a cleaning solution, it does not ensure complete cleaning of adherent debris. Complete cleaning only occurs when the adhesion of the debris is overcome by shear stress. Additionally, an effective cleaning method may act to weaken the adhesive bond between the debris and the surface to which it adheres to reduce the required stress. The adhesive strength must be overcome by a significant margin to ensure complete cleaning. Total coverage of all surfaces by shear stress is required and sufficient mass transfer must be provided to prevent loosened debris from shielding unloosened debris. Simple spraying does not ensure that these conditions are met.
It can be estimated that prior art techniques which use a total of only a drop or two of liquid would not provide enough liquid for the surface area of a dental handpiece tube to achieve significant re-formation of droplets. One or two drops equals tenths of a milliliter. The present method uses a continuous flow for a period of time such that the amount of liquid used for the same purpose would be tens of milliliters, some two orders of magnitude higher.
In addition to biofilm, passageways of various medical devices may contain food particles, particles of various bodily tissues, mucous, saliva, unclotted or clotted blood or blood components, pathogens, macromolecules and the like, which are referred to hereinafter as xe2x80x9cdebrisxe2x80x9d. It is also necessary to remove this debris from the passageways in which it exists. Such debris may even need to be cleaned from passageways which are not fluid-carrying passageways in the normal use of the device, such as where a cable slides inside a sheath or conduit in an endoscope or biopsy device. Infections arising from the use of endoscopic devices have been reported and traced to the inefficient cleaning and debris removal by conventional methods.
Endoscopes may contain a passageway for use of a biopsy device, as well as passageways for other purposes. Both the internal passageways and the exterior of the endoscope must be cleaned after each use. The biopsy device itself also has interior and exterior surfaces which must be kept clean. Guidelines for cleaning gastrointestinal and other flexible endoscopy units promulgated by the American Society for Gastrointestinal Endoscopy and other bodies include a multi-step method for cleaning tubing between uses to prevent cross-infection between patients. First, mechanical cleaning using a brush and a detergent solution is performed soon after use. The tubing is then rinsed with water and then a disinfection is carried out using a liquid chemical disinfectant such as a gluteraldehyde solution. The tubing is then rinsed with sterile water and dried with forced air. However, this method is time-consuming and suffers from inefficiency in removing all pathogens and other debris, as well as being subject to variations in technique from one operator to another.
In devices such as heat exchangers, there is a need to remove biofilm, algae, mineral deposits or corrosion products, the last two being referred to as scale, from their surfaces. Such substances decrease the thermal efficiency of heat exchangers.
There are also applications for the cleaning of fluid passageways whose walls are permeable. Surfaces which are permeable or porous are frequently described as membranes. Herein, the term membrane is used to denote porosity and permeability for a surface of any geometry, and most commonly a geometry which is of a tubular shape or other shape more complex than flat, such as a hollow fiber filter or a hemodialyzer or a spiral wound filter. Applications in which the wall of the fluid passageway is a permeable membrane include microfiltration, ultrafiltration, kidney dialysis, reverse osmosis and the like. In such applications it is necessary to remove from the membrane such contaminants as small particles of any undesirable substances, large molecular weight macromolecules, biofilm, and (in the case of hemodialyzers) adsorbed serum proteins, blood cells, cell fragments, platelets, salts and other soluble or dispersed blood constituents. All of these are included in the term xe2x80x9cdebrisxe2x80x9d. Cleaning permeable membranes is more difficult than cleaning solid surfaces, because whatever is held back by the membrane can lodge either immediately at the membrane exposed surface or within the membrane pore structure, with the surfaces within the membrane pore structure being more difficult to clean.
At present hemodialyzers are typically re-used up to about 30 times. However, for some patients, who may represent roughly one-quarter of hemodialysis patients, hemodialyzers clog more quickly and thus can only be re-used three or four times. A better method of cleaning and disinfecting hemodialyzers between uses could extend their useful life, with consequent economic savings, and possibly improve the biological performance of re-used hemodialyzers. Even if the improved cleaning only extended the life of those hemodialyzers which are presently re-used three or four times up to re-use of up to 15 times, the economic savings would be considerable.
Membrane filters, at present, are cleaned with harsh liquid-phase chemicals and/or large quantities of hot water, including backflushing. Even though such membranes are cleaned at regular intervals, they never return to their original flux levels. Essentially, this constitutes a permanent de-rating of the membrane""s capacity.
In all of these applications and geometries, better cleaning methods for passageways would be useful to more completely and easily remove the biofilm, debris, contaminants and the like. In any filtration application an improved cleaning method would either extend its membrane life or improve the performance of the processing.
For medical/dental applications a thorough cleaning is a very important first step in disinfecting or sterilizing the equipment. A good initial cleaning makes any subsequent disinfection or sterilization procedure easier and more effective by reducing the bioburden which has to be killed during disinfection or sterilization. At present the major forms of sterilization are heat, harsh chemicals and radiation. Some medical devices contain materials or components which suffer damage from one or more of these processes, or there may be times when for other reasons it may be impractical to use them. Thus improved methods of disinfection or sterilization which stay close to ambient conditions, use benign chemistry, and are simple to perform would be broadly useful for many medical and industrial applications. Thus, improved methods of cleaning regular and irregular surfaces and passageways of various medical devices, as well as devices in contact with food or potable water, or those that need to be made sterile, methods that can be carried out rapidly, effectively and inexpensively, and that do not employ extreme temperatures, harsh or toxic chemicals or radiation, would be highly desirable.
In accordance with the method of the present invention, a mixture of gas and a suitable liquid, preferably including one or more cleaning agents, is used to create a mixed-phase flow along a surface, which creates shear or impact stresses or similar conditions sufficient to remove biofilm, debris and contaminants from their surfaces. The cleaning agent is commonly a surfactant, but may also be or include an oxidizing agent, an alcohol, a non-surfactant detergent or a solid material. The method may be applied to passageway geometries of considerable complexity, including surfaces made of a porous membrane. It further includes optimally varying parameters such as the fluid mechanics regime of the mixed-phase flow, the chemistry of the cleaning liquid, temperature, and, in the case of membranes, the direction, magnitude and sequencing of pressure differences across the membrane.